The present invention relates to controls for discharge lighting.
Discharge lamps have a high impedance before they are lit, and a low impedance when they are lit. A disadvantage of these lamps is that additional components are required for their proper operation. In particular a means of limiting or controlling the current drawn by the lamp is required, together with a means of starting an arc discharge in the lamp.
The arrangements of components associated with the control of lamp current are commonly referred to as a xe2x80x9cballastxe2x80x9d, whilst those components performing the function of starting the lamp are collectively referred to as the xe2x80x9cignitorxe2x80x9d.
Traditionally the ballast function was performed by a large and heavy inductor operating at the power line or source frequency, whilst the ignitor function was performed by the generation of high (2-5 kV) voltage pulses superimposed onto supply terminals of the lamp.
Electronic control means may be used to overcome the above mentioned disadvantages, but as a result of their complexity, new disadvantages of cost and reliability have prevented their widespread use. Electronic controls frequently operate at a high frequency, much higher than the power source frequency. However, a problem exists with known designs of electronic controls in that source frequency power is present at lamp terminals provided for connection of a discharge lamp. The presence of source frequency power at lamp terminals presents a potentially fatal electrical shock hazard when changing lamps. There is a particular risk when a lamp fails by fracturing of a protective glass envelope containing light emitting elements and exposed electrical conductors. To reduce the risk it is known to fit a safety switch on a luminaire, such that when the luminaire is opened to gain access to the lamp, the safety switch ensures that power to the lamp is disconnected.
A problem with known designs of electronic controls is that un-necessary stress is caused to the electronic control and other components of the luminaire during the normal starting procedure or when a bulb fails or is absent from the luminaire. A primary cause of this stress arises from inappropriate operation of the control as a consequence of uncertainty as to whether the lamp is lit or not.
British patent GB 2,177,309 (Erbe Elecktromedizin) discloses a surgical instrument requiring a low power output, comprising a high frequency power source for powering a tool with high frequency and having two capacitors in series with the tool. The capacitors have values such that their impedance is small for the high frequency current of the power source and high for possible low frequency leakage currents flowing to ground arising from contact by a patient with some other defective electrical appliance. The capacitors do not provide the desired level of protection, hence a safety circuit is arranged to trip a safety switch, when the low frequency leakage current exceeds an adjustable limiting value, so as to disconnect the high frequency power source from ground. Disconnection of the power supply from ground does not prevent hazards from any other inadvertent contact between the patient and ground. The publication describes a power supply for a surgical instrument that would not be suitable for use with a discharge lamp which has to provide high ignition voltage and lower operating voltages across a lamp.
U.S. Pat. No. 5,625,258 (Preis Karl-Heinrich, assigned to Bosch) discloses a power supply having a low frequency output, with a frequency of several hundred hertz, for powering a discharge lamp. This publication states that xe2x80x9cswitching off of the power supply such that the maximum voltage which occurs is reduced at all possible contact points to values which do not present a hazard is essential.xe2x80x9d Hence, to ensure safe operation a fault current detection means is provided to emit a switch off signal to inhibit the operation of a high frequency voltage converter when a fault current having a DC or low frequency at that of an inverter 6 is detected above a pre-set threshold. This publication is not relevant to the use of a high frequency output for powering a discharge lamp, or the use of a capacitor in series with an output terminal of a power supply for a discharge lamp to provide a supply where a fault current cannot exceed a safe level. A disadvantage of the power supply described in this publication is that the lamp is extinguished when a fault current is detected. A further disadvantage is that a person inadvertently touching a live part is subject to a potentially dangerous low frequency voltage.
British patent GB 2,229,873 (Koito Manufacturing Co) discloses a lighting circuit providing an alternating current output for powering a metal halide lamp, having a detection means arranged to detect the alternating current flowing through the lamp once the lamp has ignited. The detection means is either by means of a current transformer, which has disadvantages of cost, or by means of a series capacitor, which to provide a suitably low output voltage across it for the purposes of detection must necessarily have a large value of capacitance. A disadvantage of the lighting circuit disclosed in this publication is that the circuit would not provide safety to a person inadvertently touching an output terminal for supplying power to the lamp.
U.S. Pat. No. 5,965,986 (Bosch) discloses an arrangement for detecting the ignition of a high pressure gas discharge lamp by detecting a sudden decrease in the high voltage applied across a lamp to effect ignition. A disadvantage of this arrangement is that it can only detect the initial ignition of a lamp, and cannot detect a lamp that has initially ignited, but then extinguished. The circuit disclosed in this publication would be unsuitable for use with a power supply that produces a resonant high frequency alternating current voltage for igniting a lamp.
According to the present invention there is provided a control and a discharge lamp, the control arranged for connection to a source of alternating current electrical power at a source frequency, the source frequency being a low frequency, the control having a first and a second terminal for supplying an output of electrical power to operate the discharge lamp, the output power being at a high frequency, and a first capacitor being connected in series with the lamp to the first terminal, the first capacitor having a value of capacitance, characterised in that the value of capacitance is sufficiently small to limit a current flow from the first terminal at the source frequency to less than 30 mA, and wherein a second capacitor is connected in series with the lamp to the second terminal, the second capacitor having a value of capacitance, the value of capacitance being sufficiently small to limit a current flow from the second terminal at the source frequency to less than 30 mA.
A benefit of placing a suitably small capacitor in series with a lamp terminal, is that the maximum current flow at the source frequency from that terminal is limited to a value of 30 mA which is accepted as being a maximum safe current that is unlikely to present a fatal shock hazard to the large majority of persons. Hence a person who accidentally makes contact with both terminals is most unlikely to suffer fatal injuries from this maximum safe current of 30 mA. A further benefit is that the cost and complexity of a luminaire may be reduced by the elimination of a safety switch that disconnects the electrical power to the terminals within the luminaire when an openable cover is opened to gain access for the purposes of changing a lamp. Such a safety switch is relatively expensive, as it has to be of an approved type and care has to be taken in the design of its mounting to minimise the risk of a user disabling the switch. Such a safety switch, however, can provide no protection in the event that a glass window in the openable cover is broken, allowing access to the lamp which may have been damaged by an explosive failure of the enclosed lamp thereby exposing live electrical conductors that form part of the lamp.
A benefit of having a second capacitor is that a person who accidentally makes contact with one terminal and an electrical earth will be most unlikely to suffer fatal injuries from this maximum safe current 30 mA, and further this protection is irrespective of which terminal is touched or which terminal is electrically connected to a live or a neutral pole of the source.
The current flow at the source frequency may be more preferably limited to less than 5 mA or even more preferably the current flow at the source frequency is limited to less than 1 mA. A benefit of further limiting the current flow is that a potential hazard is further reduced, so that a small percentage of the population that may suffer a fatal injury from accidental electrical contact with the lamp terminals is considered to be an acceptable risk, such that such a control for a discharge lamp may be considered to provide safety isolation of the lamp terminals from the source supply. A further benefit is that it may be possible to omit an earth connection to such a lampholder, saving costs and complexity.
Preferably the output of electrical power is an alternating current supply of a frequency greater than 100 kHz, and more preferably the frequency is greater than 200 kHz. A benefit of such a high output frequency is that a capacitor with a sufficiently small value of capacitance to limit the current flow at the supply frequency to a safe value will conduct sufficient energy at a high output frequency to operate a discharge lamp with usefully high light output.
A further benefit of a high output frequency is that conduction at such frequencies is concentrated in the surface layers of a conductor, hence a person touching lamp terminals having an ability to supply sufficient energy at the high frequency will not suffer fatal injury, any injury from the high frequency output being limited to localised skin burns at the point of contact with the terminals.
Preferably the output of electrical power is an alternating current with a sinusoidal wave form. A benefit of using a sinusoidal wave form is that the capacitors may be of a smaller physical size, and hence cheaper, than would be necessary if a conventional square wave output were used. A sine wave voltage output produces a sine wave current output and avoids the distorted current wave form arising from the harmonics generated as a result of a conventional square wave form voltage output.
Preferably the control is arranged to apply a DC voltage to the first lamp terminal for connection to the lamp, and the control further comprising a sensor to detect the conduction of the DC voltage by the lamp, the sensor being arranged to provide a feedback signal to the control when the conduction of the DC voltage is detected.
By DC it is in the above paragraph and hereinafter meant a current having a predominant DC element, examples of which will be shown in the accompanying drawings.
A benefit of this is that when a control has attempted to start a lamp, it is possible to detect that the lamp is lit while it is in a glow mode of operation, and before an arc has been established. A further benefit is that the control may be arranged to alter an output power to the lamp according to the feedback from the sensor.
Preferably the sensor detects the conduction of the DC voltage by the lamp, by detecting a DC voltage level across a capacitor connected to the second lamp terminal.
A benefit of detecting the voltage level across a capacitor is that a high degree of immunity from interference from electrical noise may be obtained.
Preferably the sensor is connected in series with the lamp to detect the current arising from the conduction of the DC voltage by the lamp.
A benefit of detecting the current arising from the conduction of the DC voltage by the lamp is that the sensor may be connected to either the first or the second lamp terminal.
Preferably the sensor is connected to the second lamp terminal.
Preferably the discharge lamp is a high intensity discharge lamp.
Preferably the sensor detects the conduction of the DC voltage by the lamp before an arc is established in the lamp.
Preferably the control is arranged to provide an alternating current output to power the lamp with an output frequency above 400 kHz. More preferably the alternating current (A.C.) output has an output frequency above 500 kHz.
A benefit of a high output frequency is that the capacitor connected to the second lamp terminal may be of a small value of capacitance, and hence a time taken for the capacitor to become charged with the DC voltage level across the capacitor is reduced.
Preferably the control is further arranged to apply a direct current voltage to the first terminal, the direct current voltage being supplied through a resistor connected in parallel with the first capacitor, the control further comprising a sensor to detect the conduction of the direct current voltage by the lamp, the sensor being arranged to provide a feedback signal to the control when the direct current voltage is detected.
A benefit of this is that when a control has attempted to start a lamp, it is possible to detect that the lamp is lit while it is in a glow mode of operation, and before an arc has been established, and the capacitor in series with the lamp limits the amount of DC current that the lamp is subject to, hence avoiding damage to the lamp arising from the conduction of a large DC current. A further benefit is that the control may be arranged to alter an output power to the lamp according to the feedback from the sensor.
Preferably the discharge lamp is a high intensity discharge lamp. Such lamps have a high impedance before they are lit, and a low impedance while they are lit. High intensity discharge lamps are characterised by a short arc length, typically less than 5 mm for a 20 watt lamp, and have a sealed envelope containing at least two electrodes for an electrical discharge, and typically have a high internal pressure when hot.